This invention relates to ophthalmological surgery techniques which employ a laser to effect ablative photodecomposition of the anterior surface of the cornea in order to correct vision defects.
Ultraviolet laser based systems and methods are known for enabling ophthalmological surgery on the surface of the cornea in order to correct vision defects by the technique known as ablative photodecomposition. In such systems and methods, the irradiated flux density and exposure time of the cornea to the ultraviolet laser radiation are so controlled as to provide a surface sculpting of the cornea to achieve a desired ultimate surface change in the cornea, all in order to correct an optical defect. Such systems and methods are disclosed in the following U.S. patents and patent applications, the disclosures of which are hereby incorporated by reference: U.S. Pat. No. 4,665,913 issued May 19, 1987 for "METHOD FOR OPHTHALMOLOGICAL SURGERY"; U.S. Pat. No. 4,669,466 issued Jun. 2, 1987 for "METHOD AND APPARATUS FOR ANALYSIS AND CORRECTION OF ABNORMAL REFRACTIVE ERRORS OF THE EYE"; U.S. Pat. No. 4,732,148 issued Mar. 22, 1988 for "METHOD FOR PERFORMING OPHTHALMIC LASER SURGERY"; U.S. Pat. No. 4,770,172 issued Sep. 13, 1988 for "METHOD OF LASER-SCULPTURE 0F THE OPTICALLY USED PORTION OF THE CORNEA"; U.S. Pat. No. 4,773,414 issued Sep. 27, 1988 for "METHOD OF LASER-SCULPTURE OF THE OPTICALLY USED PORTION OF THE CORNEA"; U.S. patent application Ser. No. 109,812 filed Oct. 16, 1987 for "LASER SURGERY METHOD AND APPARATUS"; and U.S. Pat. No. 5,163,934 issued Nov. 17, 1992 for "PHOTOREFRACTIVE KERATECTOMY".
In the above-cited U.S. Pat. No. 4,665,913 several different techniques are described which are designed to effect corrections for specific types of optical errors in the eye. For example, a myopic condition is corrected by laser sculpting the anterior corneal surface to reduce the curvature. In addition, an astigmatic condition, which is typically characterized by a cylindrical component of curvature departing from the otherwise generally spherical curvature of the surface of the cornea, is corrected by effecting cylindrical ablation about the axis of cylindrical curvature of the eye. Further, a hyperopic condition is corrected by laser sculpting the corneal surface to increase the curvature.
In a typical laser surgical procedure, the region of the anterior corneal surface to be ablated in order to effect the optical correction is designated the optical zone. Depending on the nature of the desired optical correction, this zone may or may not be centered on the center of the pupil or on the apex of the anterior corneal surface.
The technique for increasing the curvature of the corneal surface for hyperopia error correction involves selectively varying the area of the cornea exposed to the laser beam radiation to produce an essentially spherical surface profile of increased curvature. This selective variation of the irradiated area may be accomplished in a variety of ways. For example, U.S. Pat. No. 4,665,913 cited above discloses the technique of scanning the region of the corneal surface to be ablated with a laser beam having a relatively small cross-sectional area (compared to the optical zone to be ablated) in such a manner that the depth of penetration increases with distance from the intended center of ablation. This is achieved by scanning the beam more times over the deeper regions than the shallow regions. As pointed out in U.S. Pat. No. 5,163,934, such ablations tend to be rougher than area ablations. The result is a new substantially spherical profile for the anterior corneal surface with maximum depth of cut at the extreme outer boundary of the optical zone. Another technique disclosed in the above-cited U.S. Pat. No. 4,732,148 employs a rotatable mask having a plurality of elliptical annular apertures which are progressively inserted into the laser beam path to provide progressive shaping of the laser beam in order to achieve the desired profile.
One of the major difficulties encountered in the application of laser surgery techniques to effect hyperopic refractive error corrections lies in the nature of the boundary between the optical zone and the untreated area. Since the anterior surface of the cornea is sculpted during the process to have an increased curvature, the maximum depth of cut necessarily occurs at the outer boundary of the optical zone. The generally annular region between this outer boundary and the adjacent untreated anterior surface portion of the cornea typically exhibits steep walls after the completion of the photoablation procedure. After the surgery the tendency of the eye is to eliminate these steep walls by stimulated healing response involving concurrent epithelial cell growth and stromal remodelling by the deposition of collagen, which results in corneal smoothing by filling in tissue in the steep walled region. This natural healing response acts to eliminate the discontinuity, resulting in a buildup of tissue in the steep walled region and over the outer portion of the optical zone. This natural phenomenon, sometimes termed the "hyperopic shift" in phototherapeutic keratectomy, causes a lack of precision for a given surgical procedure and diminished predictability, which tend to counteract the beneficial effects of the refractive correction procedure and thereby reduce the desirability of the procedure to the prospective patient.
Efforts have been made in the past to laser sculpt a transition zone to provide a more gradual sloping of the walls and to eliminate the sharp discontinuity between the outer edge of the optical zone and the edge of the untreated area. Efforts have included the use of a beam rotation or scanning mechanism operated by a computer to provide programmed ablation of the transition zone to achieve a sigmoidal or other profile. While somewhat effective, this technique suffers from the disadvantage of typically requiring additional optical elements (such as a rotatable off-axis mirror or revolving prism having suitable optical properties) which adds complexity to the delivery system optics commonly found in laser sculpting ophthalmological surgical systems. One specific technique, which is disclosed in published European Patent Application No. 0 296 982 published Dec. 28, 1988, employs a rotatable mask having one or more profiling apertures whose shape is designed to provide a smoother profile in the transition zone in the course of performing a specific ablation procedure. This reference also teaches the use of a rotating prism aligned along the beam axis in combination with a translatable platform bearing a focusing lens in order to both translate and rotate the aperture image along the anterior corneal surface. This technique, while considered effective for some purposes, requires a relatively complicated optical delivery system in order to provide the desired profiling. In addition, the use of mirrors and prisms in delivery system optics in laser surgery systems suffers from certain disadvantages. In particular, the addition of prisms decreases the total energy transmission of the system. Further, the reflectance of dielectric mirrors used in certain systems varies with reflectance angle, which can dynamically alter the irradiance delivered to the cornea while displacing the beam image over the cornea.
Another difficulty encountered in the application of laser surgery techniques to effect hyperopic refractive error corrections lies in the requirement for relatively large transition zones outside the optical zone. In particular, while the intended optical zone is typically on the order of about 5 mm in diameter, the outer limit of the transition zone can be as large as 10 mm in diameter. If the rotating mask arrangement described above is used to effect the ablation in both the optical zone and the transition zone, the beam diameter must be commensurate in size with the largest aperture outer diameter (i.e., at least about 10 mm). In general, the larger the beam diameter the less uniform the energy density across the beam and the less reliable the photoablation process. Further, the increased beam area requires a laser beam of substantially greater energy, which necessitates a more expensive laser. Also, the increased energy flowing through the optical components causes optical deterioration at a faster rate, thereby increasing maintenance and replacement costs.